In conventional print station apparatus, it has heretofore been proposed to provide various devices for spacing recording media relative to a print head for proper printing relationship. Typical of one of these prior expedients is to provide a gap guide means against which the recording media is urged by a platen so that an appropriate gap is provided between the print head and the recording media for proper operations of the print head. In this prior expedient, the platen is arranged to yieldingly urge the recording media against the gap guide means so as to automatically accommodate one or more layers of the recording media. This platen assembly is particularly useful in point-of-sale data terminals where the recording media is usually multiple copy forms of varying numbers for bills of sale, invoices, contracts of sale, and the like. However, problems are encountered with such prior art platen assembly in that it is difficult to correctly position the face of the recording media relative to the print head especially where the recording media is of bank passbook type documents whose thicknesses vary on either side of the centerfold.
To accommodate the thickness variations of the passbook type documents, it has also heretofore been proposed to have a platen body made of elastic or resilient soft material and formed with a plurality of alternating annular grooves and rings disposed longitudinally along the platen body. With this prior art platen body, the passbook type document is pressed by the ring portions formed longitudinally along the platen body so that areas at which the document is not urged by the platen body are provided and, therefore, the platen body can not correctly position the passbook type document relative to the print head. Another problem inherent in this platen body resides in the fact that the document can not properly be printed with uniform qualities due to the difference in forces acting on the passbook type document to support the same. In addition, the plate body is arranged to be pressed onto the passbook type document with relatively larger forces so that the ring portions deflect to accommodate the document thickness variations. This is particularly serious especially where the passbook type document is made of pressure-sensitive papers which may be contaminated with a slight pressure applied thereon. As previously discussed hereinabove, since the platen body has a plurality of annular grooves longitudinally disposed along the platen body, a relatively thin single layer of recording medium can not be printed.
In order to properly hold the bank passbook type document, it has also been proposed to have the print station apparatus equipped with a sensing means to sense the thickness of the document and a print head driving mechanism such as a servo motor or a pulse motor which is controlled in response to electric signals indicative of the thickness of the document to move the print head relative to the document. With this expedient, the print station apparatus becomes necessarily complex in construction and, therefore, the manufacturing cost increases.
A further problem encountered in the prior art printers is that it is difficult to form clear imprints of uniform density and, further, the print head is not smoothly traversed over the recording media especially in a case where the recording media is of the passbook type document in which the thickness to either side of the centerfold varies.